Place the butter in a medium saucepan over low heat. The butter will foam and then start to brown. When it is caramel colored, immediately remove it from the heat and transfer to a bowl to stop the cooking. Cool to room temperature.

Beat the cooled brown butter with the tahini and both sugars. It should become light and fluffy. Add the eggs, one at a time, and the vanilla.

Combine the dry ingredients and add to the butter mixture, mixing until they are just combined. Stir in the chocolate chips. Transfer the dough to a bowl. Cover and chill overnight.

To bake: Preheat the oven to 325. Shape the dough into 1 1/2 inch balls and place onto a parchment lined baking sheet. Flatten slightly. Bake 12-14 minutes. Cookies should be golden brown and just set. Sprinkle with fleur de sel and serve immediately.

The dough will keep in the refrigerator for about a week. To freeze uncooked dough, shape into balls and place on a parchment lined baking sheet. Freeze until solid and then transfer to a ziplock bag. To bake, let the dough stand at room temperature while the oven heats. Cook as above, but add 1-2 minutes, checking toward the end.